1. Field of the Invention
The present invention relates to an image reading apparatus and, more particularly, to an image reading apparatus which uses photoelectric conversion elements, as image reading elements, of a type divided into the first half portion and the second half portion in the main scanning direction.
2. Description of the Related Art
Conventionally, standalone image scanners and the image reading apparatuses mounted in digital copying machines and the like use CCD linear image sensors and CMOS linear image sensors. For the sake of high read rates, these image sensors use a technique called the ODD/EVEN output method which alternately allots outputs according to odd-numbered/even-numbered pixels or a method of dividing outputs into the first half portion and the second half portion in the main scanning direction (see Japanese Patent Laid-Open No. 11-261760). There has also been proposed an image sensor which can implement double rate reading of an ODD/EVEN 2-channel output image sensor (see Japanese Patent Laid-Open No. 11-215298). Such a sensor performs 4-channel output operation by combining ODD/EVEN method and the method of dividing outputs into the first half portion and the second half portion.
A conventional CCD will be described below. FIG. 19 shows the arrangement of the conventional CCD sensor. Reference numeral 201 denotes the photoelectric conversion element of the CCD sensor, i.e., a photo-detector (to be referred to as PD hereinafter) unit constituted by PDs. OB pixels OB1 to OB64 on which light-shielding films are formed to block light and are designed to detect the reading levels of the PDs at a dark time are arranged on the two end portions of the PD unit 201 such that the 32 pixels are arranged on each of the right and left end portions. PDs S1 to S7680 located inside the positions of the OB pixels are PDs for reading light reflected by a document. The PD unit 201 is divided into right and left portions at the central portion between the PDs S3840 and S3841. A shift gate (1) 202, a shift gate (2) 203, a shift gate (3) 204, and a shift gate (4) 205 are circuits to transfer the charges generated in the respective PD elements upon photoelectric conversion in the PD unit 201 to a CCD shift register unit as a charge transfer element. The shift gate (1) 202 transfers charges in the right odd-numbered (ODD) portion of the PD unit 201 to a CCD analog shift register (1) 206. The shift gate (2) 203 transfers charges in the right even-numbered (EVEN) portion of the PD unit 201 to a CCD analog shift register (2) 207. The shift gate (3) 204 transfers charges in the left odd-numbered (ODD) portion of the PD unit 201 to a CCD analog shift register (3) 208. The shift gate (4) 205 transfers charges in the left even-numbered (EVEN) portion of the PD unit 201 to a CCD analog shift register (4) 209. The respective CCD analog shift registers send the received charges to output buffers 210 to 213 pixel by pixel by the charge transfer scheme while sequentially shifting the charges to adjacent pixels. The amplifier circuits in the output buffers convert the charges in the respective pixels, which are sent to the output buffers by the charge transfer scheme, into desired voltages and output them as OS1, OS2, OS3, and OS4.
FIG. 20 shows the state of a conventional image data rearrangement. FIG. 20 shows an arrangement order 905 of image data DS1 to DS4 output from CCD analog shift registers 901 to 904. In this manner, the image data are sequentially rearranged from the end portions (S1 and S2, and S7670 and S7680) of the PD unit 201 toward the central portion. Finally, the rearrangement at the central portion ends. After the rearrangement is complete in this manner, the charges pass though a shading correction circuit 804 described above and a 10 bit→8 bit conversion circuit 805. As shown in FIG. 20, only an effective pixel area is extracted from the charges by the main scanning valid pixel signal generated by a reading control IC 702 to output a Video_OUT signal. That is, in the arrangement of a conventional CCD sensor, the CCD analog shift register units sequentially transfer charges from the OB pixels located at the outermost portions to the output buffers. Finally, the data of the pixels located at the central portion (S3839 for OS1, S3840 for OS2, S3841 for OS3, and S3842 for OS4) are output.
FIG. 21 is a timing chart showing image data output control by a conventional CCD sensor. A Low interval of a light receiving time control signal is a time during which the PD unit 201 on one main scanning line is receiving light. A Hi interval of this signal is a time during which the charges generated in the PD unit is transferred to the shift register unit by gate shifting. A charge transfer time is a time during which the CCD analog shift register unit transfers one-line charges. In addition, “Video Output Signal” represents the timing at which a Video output is output after image processing. As shown in FIG. 21, the conventional technique is characterized in that since the entire effective image area is not read unless a charge transfer time elapses, the output timing of a Video output signal is after the end of charge transfer.
FIG. 22 shows the image processing blocks of an image processing unit 2505 in a conventional CCD sensor. This arrangement is the same as that of an image processing unit 705 according to an embodiment (to be described in detail with reference to FIG. 5) except that a rearrangement unit 803 of the conventional image processing unit includes a line memory 807. This is because, since image data cannot be supplied to the subsequent image processing block until the rearrangement of the image data is complete for the correction of the step between the right and left portions, it is necessary to temporarily store the data in the memory.
A conventional CCD linear image sensor of a type divided into the first half portion and the second half portion, however, needs to output signals by sequentially transferring signal charges from the pixels on the right and left end portions in the main scanning direction to an output buffer because of the structural problem of the CCD in terms of the transfer of signal charges. Dividing the CCD into the first half portion and the second half portion will shorten the transfer time from the CCD to half. However, image data at a portion corresponding to the joint portion at the central portion are transferred last. Assume that it is necessary to perform image processing such as correcting variations in the right and left CCD device portions on the joint portion at the central portion and filtering a plurality of pixels including pixels on the join portion. In this case, it is impossible to perform image transfer to the next image processing unit before the end of processing on the joint portion. The CCD which is divided into two portions outputs signals by sequentially transferring signal charges from the pixels on the right and left end portions in the main scanning direction to the output buffer. This CCD outputs data by signal charges from the outermost portions which are not an effective image, and hence it is always necessary to read all image signals. That is, even when reading a small image projected on the central portion of the CCD, it is always necessary to read all pixels. This makes it impossible to change the read time in accordance with the document size.